Today there are many devices, especially measuring instruments and sensors, in which piezoelectric elements are used as electromechanical transducers.
There are e.g. fill level limit switches commercially available, which are used for limit level detection, overfill prevention or protection against pumps running dry. These have e.g. a mechanical oscillation structure extending into a container. The structure is caused to oscillate by means of a piezoelectric element or by means of a plurality of piezoelectric elements arranged in a stack. The resonance frequency and/or the amplitude of an excited oscillation is registered, and from this it is determined whether the oscillation structure is oscillating freely or, instead, covered with a fill material.
The piezoelectric elements are usually provided with electrodes and are connected by means of electrical contact flags or lugs. The contact flags are connected with an electronic circuit by way of electrical conductors, through which the piezoelectric elements are either excited to oscillate, or their oscillation-dependent signals are registered.
Examples of electromechanical transducers with one or more piezoelectric elements are described in EP-A 875 742, EP-A 875 741, EP-A 875 740, EP-A 875 739 and European Patent Application 10028319.5 filed 7 Jun. 2000.
Presently, the contact flags are adhesively attached to the electrodes. Adhesives are always applied onto a flat area of the contact surfaces of the electrodes, or of the contact flags, as the case may be. The thickness of the adhesive layer depends in such practice on the amount of adhesive applied, the contact pressure under which the parts to be bonded are brought together, and the, in part, also temperature-dependent viscosity of the adhesive before it sets. The finite thickness of the adhesive layer means that there will be a gap between the electrode and the contact flag. Such a gap occurs naturally also in the alternative practice where the contact flag is provided with solder dots and the electrode is soldered on.
With time, the adhesive experiences aging effects, which can affect the contact reliability of the connection.